Principle of Residual Chlorine Detection by Spectrophotometry

Residual chlorine is widely used in drinking water and swimming pools to maintain disinfection. Its concentration must be monitored regularly. Spectrophotometry offers a simple, fast, and reliable way to measure residual chlorine. This article explains the principle behind this method without using chemical formulas.

Basic Principle

The spectrophotometric method relies on a colour?forming chemical reaction. A reagent is added to the water sample. If residual chlorine is present, it reacts with the reagent and produces a coloured compound. The intensity of this colour is directly proportional to the chlorine concentration.

The most common reagent used is DPD (N,N?diethyl?p?phenylenediamine). When DPD is added to a water sample containing free chlorine, the solution immediately turns a shade of pink or magenta. The stronger the chlorine, the deeper the colour.

How Measurement Works

A spectrophotometer shines a beam of light of a specific wavelength through the coloured solution. For the DPD reaction, the wavelength is typically around 515 nm (visible light region). The instrument measures how much light is absorbed by the solution.

The more chlorine present, the darker the colour, and therefore the more light is absorbed. By comparing the absorbance of the sample to a set of known standard solutions, the exact chlorine concentration can be determined. This is usually done using a calibration curve.

Measuring Different Forms of Chlorine

The basic DPD test measures free chlorine (hypochlorous acid and hypochlorite ion). If potassium iodide is also added to the DPD reagent, the method measures total residual chlorine, which includes free chlorine plus combined chlorine (chloramines). By measuring both free and total chlorine, the amount of combined chlorine can be calculated by difference.

Why It Is Useful

This method is sensitive down to very low chlorine levels (around 0.01 mg/L). It is easy to perform with portable spectrophotometers, making it suitable for field use as well as laboratory analysis. It avoids the subjective errors of visual colour comparison kits and provides a precise, quantitative result.

Limitations

Strong oxidants such as ozone, manganese, or oxidized iron can interfere and give false high readings. Turbidity or natural colour in the water must be corrected, typically by using a sample blank. pH control is also important because the DPD reaction works best near neutral pH.

The spectrophotometric determination of residual chlorine using the DPD reagent is based on a simple colour reaction: chlorine turns the DPD reagent pink, and the darkness of the pink is measured by light absorption. This principle allows accurate, rapid, and reliable monitoring of disinfection levels in water.